In general, sludge is discharged from a sewage-treatment plant in a state having a solids content of about 2 weight %. In a conventional method for reducing the amount of such discharge, the sludge is guided to a digestor chamber for methane fermentation.
In such a conventional method, however, a digestor chamber having a large capacity is required since the rate of fermentation is slow and the sludge concentration is very dilute. Further, the conventional method is inferior in economic efficiency since the sewage gas yield thereof is 40 to 50%, at most, with the result that a considerable part of the sewage gas generated through digestion is employed in heating the sludge.
In improving sludge digestion techniques, a first important goal is to reduce the size of the digestor chamber, and a second goal is to improve the heat balance, as well as the sewage gas generation efficiency, in the process. An attempt has recently been made to enrich the sludge for increasing its solids content from about 2 weight % to 4 to 6 weight %, in order to reduce the capacity of the digestor chamber and to improve the heat balance so as to be able to recover energy through a sewage gas power generation system. However, sufficient economic efficiency cannot yet be obtained through such enrichment, and further improved performance is desired.
Two methods are generally known for improving the efficiency of methane fermentation. The first method is generally referred to as "thermophilic digestion" wherein the methane fermentation is performed at a relatively high temperature greater than about 50.degree. C., i.e., in the range of from 50.degree. C. to 55.degree. C. The fermentation temperature can be as high as about 65.degree. C., although higher temperatures are avoided in order to prevent destruction of the inoculant which is employed for methane fermentation. The thermophilic digestion method is disclosed by Hobson et al, Methane Production From Agricultural and Domestic Wastes, Applied Science Publishers, Ltd. (1981), pages 190, 226-228. In the thermophilic digestion method, the speed of fermentation is increased from 2 to 2.5 times that in ordinary methane fermentation conducted at a temperature of, for example, 35.degree. C.
The second of the generally known methods for improving the efficiency of methane fermentation is referred to as "thermal pretreatment" wherein heat treatment of raw sludge is performed prior to methane fermentation. The thermal pretreatment provides increased efficiency in methane fermentation by converting organic substances which are not easily decomposed into more biodegradable organic forms. The thermal pretreatment involves heating the raw sludge to temperatures greater than about 50.degree. C., and often temperatures in the range of from about 60.degree. C. to about 180.degree. C. are employed. Thermal pretreatment methods are disclosed by Hobson et al at page 188, Haug et al, "Effect of Thermal Pretreatment on Digestibility and Dewaterability of Organic Sludges," Journal of Water Pollution Control Federation, 50 (1), January 1978, pages 73-85, and Hiraoki et al, "Highly Efficient Anaerobic Digestion With Thermal Pretreatment," Water Science and Technology, Volume 17, Amsterdam (1984), pages 529-539. For example, Haug et al disclose that methane production increases 14% with the use of a thermal pretreatment at 100.degree. C. and 60 to 70% with a thermal treatment conducted at 175.degree. C. Hiraoki et al also disclose that gas production yield increases more than 30% with the use of a thermal pretreatment at a temperature of from 60.degree. to 80.degree. C.
Thus, the thermophilic digestion and thermal pretreatment methods are effective in improving the efficiency of sludge digestion. However, if the concentration of the sludge is from about 4 to 6%, the heating temperature is limited to about 30.degree. C. since excessively large quantities of heat are required for further heating. Thus, methane fermentation cannot be advantageously conducted at a temperature of 50.degree. to 55.degree. C. as required in the thermophilic digestion and raw sludge cannot be advantageously heated to temperatures greater than 50.degree. C. as required in the thermal pretreatment method in ordinary sewage treatment plants since excessively large quantities of heat are required for such treatments.